Field of the Invention
The invention relates to a radio-frequency signal generator. The signal generator is used for producing radio-frequency signals for digital communications equipment using continuous phase modulation.
Equipment which interchanges digital information through electromagnetic waves requires radio-frequency signals. The information is modulated onto the radio-frequency signals using suitable modulation methods.
In cordless telephones based on the DECT standard, a digital bit stream with a bit rate of 1152 kbit/s is produced as the information signal, which is also referred to below as an input signal. The abbreviation DECT stands for Digital Enhanced Cordless Telephone. A DECT network is a microcellular, digital mobile radio network for high subscriber densities. The DECT standard specifies the following layers: physical layer, medium access control layer, data link control layer and network layer. Further, the DECT standard describes a transport layer, a session layer, a presentation layer and an application layer. The physical layer divides the radio spectrum into physical channels. In addition to time and frequency, the dimension space and code are available. A TDMA method (Time Division Multiple Access) is used with 10 carrier frequencies. Those are in the frequency band from 1880 to 1900 MHz. At each carrier frequency, the TDMA structure has frames which are 10 ms long and each contain 24 time slots. One frame is transmitted every 10 ms for 470 .mu.s. That results in a bit stream with a gross data rate of 1152 kbits per second. The carrier signal separation is 1728 MHz.
A publication entitled "An Integrated Bipolar Transmitter for DECT", 1997 IEEE, by S. Heinen, discloses a transmitter essentially including a tracking synchronization system, referred to below as a Phase-Locked Loop (PLL), with an output stage and a signal stage for the digital input signal. In a time period before a transmission interval, the PLL locks onto a nominal frequency, the control loop is opened when transmission begins, and the voltage-controlled oscillator (VCO) is modulated with the shaped digital signal. The output signal from the VCO is amplified in the output stage and transmitted by an antenna. Upon reception, the PLL forms a radio-frequency signal for mixing the input signal. In that case, the control loop of the VCO is not opened. All of the components in that circuit, with the exception of the dividers in the PLL, have an analog structure. Large-scale integration into a digital CMOS process is not possible.